1. Field of the Invention
The field of the invention relates generally to composite structure manufacture, such as the manufacture of structural composite beam preforms that may be used, for example, in structures such as vehicle chassis, building wall construction, trusses, bridge construction, wall panel construction, boat structures and other structural applications. The structural composite wet out system of the invention provides significant improvements over the state of the art by enabling faster structural composite production run rates, more efficient structural cross sections resulting in lighter weight and more efficient structure, use of a wider variety of resin cure methods leading to the ability to better select resins for use in composite structures, and easier handling of a wetted composite structure immediately after wet-out and/or resin cure.
2. Background of the Invention
Structural composites have been used as components of structures in various structural applications for a number of years. Such applications may provide significant advantages over the use of metal structural elements such as, for example, elimination of corrosion, the ability to form and create specific shapes, weight reduction and the like. Typically, these composite structures, if desired to be formed as a beam or other shape of constant cross section, have been manufactured by a pultrusion process in which a die is utilized to shape and wet the structural component. In a typical pultrusion process, reinforced fibers are pulled through a resin bath in a first impregnation step, which first step may be followed by a second step of passing the wetted, un-cured composite material through a series of custom tooling. This custom tooling helps arrange and organize the fiber into the desired shape, while excess resin is squeezed out, also known as “debulking.” This custom tooling may be known as a “pre-former.” Often continuous strand mat and surface veils are added in this step to increase structure and surface finish. Once the wetted, resin impregnated fiber is organized and removed of excess resin, the composite may pass through a heated steel die, where the resin undergoes polymerization. The result of the pultrusion process is, generally, a composite structure profile of constant cross section typically comprised of Fiber Reinforced Plastic (FRP). Many pultrusion systems are limited to the use of thermoset resins due to the utilization of heated die to achieve polymerization of the resin.
The pultrusion systems of the prior art may be utilized to produce FRP composite structures of constant longitudinal cross section that may be of a solid cross section, or a cross section that contains voids (or hollow spaces) running lengthwise along the pulling, or longitudinal, axis of the pultruded composite structure. However, it may be desired in many structural applications to create structural members comprising, generally, a structural foam core interior surrounded by an outer reinforcing fabric layer or layers. It may further be desired that the outer reinforcing layers may further be impregnated with a resin, which is typically limited to a theromoset resin. Pultrusion systems are not readily adapted to produce structural foam core interiors surrounded by an outer reinforcing fabric layer or layers. Pultrusion systems are therefore limited in their application in this manner as well. It may also be desired to produce composite panels comprising internal structural members that are oriented transverse to the longitudinal axis of motivation, for example a continuous wall panel with structural members in the transverse direction, which is particularly useful for some applications such as trailers, buildings, shelters and the like. The pultrusion systems of the prior art are not able to produce panel with structural members oriented transversely to the direction of pull (or longitudinal axis of motivation) of the pultrusion as is the present invention.
The pultrusion process is further generally limited to the use of thermoset resins, which is a significant drawback to the use of pultrusion. For example, it may be desired that the particular resin chosen for the fabrication of a composite structural member be chosen based not on its availability as a thermoset resin, but rather on its chemical or structural properties as applied to the anticipated use, load, environment, temperature, expected life and other parameters for the anticipated application. The best choice of resin for any particular use or application may simply not be available as a thermoset resin. This means that there are certain desirable resins, for instance light curable resins, that cannot be used in the pultrusion process and is a distinct disadvantage of the pultrusion systems of the prior art.
Regarding structural foam cores for composite structures, there exist systems and methods in the background art that teach processes and devices for the construction of such un-wetted foam core composite structures. U.S. Pat. No. 5,429,066 to Lewit et al. (hereinafter “the '066 patent”) discloses a composite structure and method of manufacturing same. Composite structures manufactured in accordance with the '066 patent have met with substantial commercial success due to their superior structural characteristics and ability to simplify the fabrication of a number of articles such as boats and other reinforced plastic structures which are manufactured using similar techniques.
The composite structure disclosed in the '066 patent is generally comprised of a structural foam core interior surrounded by an outer reinforcing fabric layer. A non-woven fabric layer, such as a mat fiber layer, may be attached to the reinforcing fabric layer. A structural foam is typically, but not necessarily, attached to the non-woven fabric layer on the side of the non-woven fabric layer opposite the reinforcing fabric by filling the interstices (pores) of the non-woven fabric layer.
Structural foams are commonly formed using two or more component parts which are mixed together immediately prior to the time that the foam is to be used. In some instances structural foam may be self-curing. For example, the structural foam may be a two part, self-expanding, self-curing urethane foam. The component parts are generally mixed together, either in a mixing fixture or in a container, prior to use. Subsequently the foam is deposited in a mold and allowed to cure. The component parts typically comprise a blowing agent which is combined with a resin.
One important factor which must be carefully monitored when manufacturing foam core composite structures is the mass ratio of component parts of the structural foam. If the mass ratio is incorrect, the structural integrity, stability, and water resistance characteristics will be undesirably altered. Due to variations in the consistency and viscosity of the constituent foam parts, it is often difficult to ensure consistent mixing of such parts in a proper mass ratio. In the case of composite structures requiring the injection of large amounts of foam in a mold, this does not create a substantial problem because the consistency and viscosity do not vary as much with high flow rates and are averaged out over time.
However, where small amounts of foam are used, foam component ratio variations can create a serious problem. In a continuous foam core production process as described herein, a second factor which must be carefully controlled is the total foam mass injected. If excessive amounts of foam are injected, the foam will have an undesirable tendency to expand through the non-woven fabric layers and into the reinforcing fabric layers when it is used for production of composite structures as described in the '066 patent.
A common type of structure which is fabricated using the techniques described in the '066 patent is an elongated beam or stringer (hereinafter “stringer”) which may be formed with various cross-sectional profiles. Such stringers are commonly used as structural elements, for example, in boat construction and as component parts in many other larger fiber reinforced plastic structures which are manufactured using similar techniques. One method of manufacturing such elongated stringers involves use of elongated molds which can be lined with fabric layers as described above. The molds are then injected with structural foam which has been formed by mixing the proper ratio of constituent parts.
Due to the rather time-consuming process of forming stringers using elongated molds, it would be desirable to provide an apparatus capable of continuously producing a length of composite stringer, such as those which are described in the '066 patent. However, in order to manufacture a composite structure in this manner, careful control must be maintained over the instantaneous mass ratio of the component foam parts as well as the total instantaneous mass of foam injected. Particularly in those instances where the cross-sectional profile of the part defines a relatively small area, the rate of foam injection may be too low to ensure that any variations in the mass ratio of the constituent foam parts are averaged out over time.
Moreover, in the case of self-expanding foam of the type used in processes such as that taught in the '066 patent, at least one of the component foam parts is a blowing agent (such as nitrogen and HCFC's) combined with a resin, which must be maintained under pressure prior to use. The resulting component is a foamy, frothy mixture that is difficult to dispense accurately in terms of mass and volume. In fact, equipment of the prior art has generally been found to be capable of providing adequate control over foam component mass ratios only at flow rates above three pounds per minute when using pressurized foam.
It can easily be seen that the pultrusion systems of the background art have significant drawbacks. First, they are slow, with run rates on the order of 2 to 3 inches per minute. Secondly, they typically are limited to heat-cure, or thermoset, resins and do not allow for all types of resin curing; for instance, ultraviolet (UV) cure may not be utilized in some cases due to the pultrusion die covering the resin and also due to the thick cross sections required because the sections a pultruded product are typically hollow, requiring a thicker wall section than would be required if the beams were filed with a structural foam such as described above. Thus, thirdly, the pultrusion process usually results in thicker, less efficient wall sections than are desired which means the finished structural composite member formed by pultrusion is heavier and more expensive than it needs to be for a given application.
The system and method of the invention described in detail below allows fabrication of a more efficient structural component, especially when the present invention is used with a structural composite preform fabrication system that comprises continuous feed production, by enabling continuous-feed production of composite structures. The composite structures produced by the process and method of the present invention are lighter, of thinner cross section, more rapidly produced, and less costly that structural members produced by the pultrusion systems of the prior art.